Electrically active impurities such as foreign atoms, native defects of the crystal or the complexes thereof can all exert a significant influence on the electrical and optical properties of semiconductor materials as well as of structures and devices made of such materials, therefore the examination of such impurities forms an indispensable method both of the research of semiconductor materials and of the quality control of manufacturing processes of active elements in the field of microelectronics.
For investigating semiconductor materials and controlling active elements the sensitivity limit for detecting electrically active impurities should be at least 10.sup.10 atoms/cm.sup.3. At present such a high sensitivity can be obtained by measuring a particular single process only. In such a process a space charge layer is provided in the semiconductor under test which can be realized by depositing a suitable metal layer (i.e. forming a Schottky diode), by forming a p-n junction or by establishing a MOS structure, etc. In case of a reverse bias the space charge layer has insulating properties i.e. it does not comprise free carriers. Those portion of electrically active defects which fall within the space charge layer can be either in an electrically active or neutral state. In the process in question the active defects are filled with free charge carriers and the subsequent thermal emission recovery process is examined.
In a known examination method of this process the structure is electrically short-circuited and cooled down from room temperature to the temperature of liquid nitrogen, and in this latter temperature a reverse bias is applied to the sample. The electrically active defects remain saturated with free charge carriers that correspond to a non-equilibrum thermal state. The time constant of the recovery process to the thermal equilibrum is EQU .tau..sub.n.sup.-1 =e.sub.n =N.sub.c .sigma.v exp (-E.sub.T /kT) (1)
wherein
.tau..sub.n is the time constant of the thermal emission process PA1 e.sub.n is the probability of the thermal emission PA1 N.sub.c is the density of the state in the valence band PA1 .sigma. is the capture cross section PA1 v is the thermal drift velocity PA1 E.sub.T is the activation energy of the electrically active defects in electronvolt units PA1 k is the Boltzman constant PA1 T is the temperature expressed in Kelvin. PA1 C.sub.m is the capacitance value detected by the capacitance meter PA1 C.sub.i is the actual capacitance of the sample PA1 R.sub.i is the serial resistance of the sample PA1 .omega. is the frequency of the capacitance measurement. PA1 N.sub.D is the shallow level dopant concentration PA1 .DELTA.C is the capacitance change PA1 C.sub.o is the capacitance of the sample.
In low temperature this time constant can be even some years long. It follows from equation (1) that the time constant decreases exponentially with increasing sample temperatures, and the thermal emission takes place when the characteristic temperature associated with the electrically active defects has been reached. The free charge carriers released during such emission can be detected by means of conventional current measurement or by detecting the changes of the capacitance of the sample. Corresponding known experimental methods are as follows:
Thermally Stimulated Current i.e. TSC is described e.g. by R. H. Bube: `Photoelectronic Materials and Devices` Ed. S. Larach pp. 100-139, 1965 (D. Van Nostard Comp.). PA0 Thermally Stimulated Capacitance, i.e. TSCap is described e.g. by Carabelles at al: Solid-St Communication 6, 167, 1968). PA0 the maximum sensitivity N.sub.T /N.sub.D .gtoreq.10.sup.-6 which corresponds to a detecting limit of N.sub.T .gtoreq.10.sup.10 atoms/cm.sup.3, PA0 the maximum coverage range of the repetition frequency of the exciting pulses is about 10.sup.5, PA0 the minimum width of the exciting pulses is 1-2 ns, PA0 the highest tolerable leakage current is 1 .mu.A (in case of maximum sensitivity), PA0 the highest measuring frequency is 1 MHz, PA0 the highest series resistance of the sample is about 100 ohm.
Another widely used way of examining the thermal emission process is represented by the transient measurement technique. In such technique the sample under test is reverse biased at predetermined constant temperature and short circuited during perodically repeated short intervals. During the short-circuiting intervals the defects are filled with free charge carriers and during re-establishment of the reverse bias a thermal emission recovery process is started which has the characteristic time constant defined by the equation (1).
The filling of defects can be made not only in an electrical way by short-circuiting the sample but also by optical excitation, by an electron beam or by means of other kinds of ionizing radiation. The excitation should be, however, periodically repetitive.
The thermal emission can be detected from the transient changes in the capacitance or current of the sample under test, see e.g. R. Williams, J. Appl. Phys. 37, 3411 (1966).
The automatic detection and evaluation of the transients have been solved by means of Deep Level Transient Spectroscopy (DLTS) technique. Such a method is described e.g. by Miller et. al: Rev. os Sci. Instrum 48, pp. 237-239, 1977 or in Hungarian Pat. No. 181.136. Owing to the possibility of their automated performance DLTS measurements have become the most widely used methods of examining thermical emission processes.
The use of DLTS technique is connected with a number of factors influencing or limiting the sensitivity, accuracy and conditions of the measurements. In the following discussion these limiting factors will be analysed in a more detailed way because the understanding of these facts is thought to be inevitable for the correct evaluation of the prior art.
In the methods based on transient capacitance measurements the capacitance represented by the sample under test is connected in a measuring bridge and excited by a high frequency signal. The examined transient capacitance is represented by a component of the high frequency output signal of the bridge which has a predetermined phase. The first limit of such measurements is formed by the upper limit of the applicable frequency. Owing to the serial resistance of the sample which is higher than zero, the following equation exists between the measured and actual capacitance of the sample: EQU C.sub.m =(C.sub.i /l+(.omega.C.sub.i R.sub.i).sup.2) (2)
in which
In everyday practice there are samples which have serial resistances higher than 100 ohms and the capacitance of such samples cannot be measured if the measuring frequency is higher than about 1 MHz. For that reason the operational frequency of the capacitance bridge in commercially used instruments is not higher than 1 MHz. In the practice, however, there exists a number of semiconductor samples with serial ohmic resistances much higher than 100 ohm and one can even meet as high resistance values as 1 Mohm. Such samples cannot be measured by DLTS methods or if still measured in that way, the sensitivity of the measurement will drop well below the required level.
In addition to the limitations caused by the presence of the series resistance the shunting effects of the leakage currents flowing in the samples represent a further limitation by decreasing the sensitivity. Owing to non-ideal surface of the semiconductor samples a certain amount of leakage current is always present when being reverse biased. If as high sensitivity is requested as mentioned hereinabove then the maximum permitted leakage current should be about 1 .mu.A. This condition cannot be satisfied easily therefore it represents a further limitation regarding the types of samples that can be examined and/or the maximum sensitivity.
Of these reasons the maximum of the measuring frequency cannot be higher than about 1 MHz which, however, limits the maximum frequency of the control pulses which alternatively excite and reverse bias the sample. In case of a measuring frequency of 1 MHz, the response time of the capacitance meter is at least 5 .mu.s long and the actual measurement cannot start before the treble of the response time has elapsed. Even if the duration of the measurement periods is chosen to be just as short as the combined duration of the exciting pulses and the subsequent dead periods, the maximum of the repetition frequency cannot be higher than about 25 kHz. This theoretical upper limit is substantially higher than the highest one of the repetition frequencies used in the practice (see Hungarian Pat. No. 182.777).
It is well known in the art that DLTS measurements are carried out generally when the temperature is varied. The changing of the temperature is disadvantageous because in addition to the comparatively long time of measurements (which can be typically between 20 minutes and two hours) it can result in the thermal treatment of the sample under test and can rearrange the structure of the defects. According to the DLTS measurement with constant phase position lock-in amplifier as disclosed e.g. in Hungarian Pat. No. 181.136 frequency-scan DLTS measurements with constant temperature can be carried out, however, the attainable frequency range is limited by the maximum frequency of capacitance measurement which is about 25 kHz and of practical considerations the lowest frequency cannot fall below 0.25 Hz. With such upper and lower limits the frequency range cannot exceed more than 5 decimal orders of magnitude.
In the paper of G. Ferenczi: `The Examination of electrically active impurities of semiconductor materials and structures` (Hiradastechnika XXXVI. 1985. 10. pp. 451-454) the frequency scan DLTS measurement is described which can offer a deep level spectrum that has an extreme value proportional with the emission time constant characteristic to the type of the particular impurity. With the attainable maximum frequency coverage of 10.sup.4 -10.sup.5 and in case of a given temperature, it is possible to detect deep levels falling in the range of activation energies between E=0.2-0.3 eV. Of that reason the practically significant range of activation energies between 0.05 eV to 0.7 eV can be covered by the frequency scan method if the measurements are repeatedly carried out in different temperatures. The corresponding temperature range is typically between 240K. and 330K. For enhancing the range of frequency scan measurements in a predetermined single temperature to cover the energies of 0.05 to 0.7 eV it is required that the frequency coverage be as high as 10.sup.11. Due to the practical limitations explained above such a wide range was not realizable. Of that reasons the examination of the full deep level spectrum inevitably required the changing of the measuring temperature that was connected with the drawbacks associated with the unwanted thermal treatment (annealing) of the sample.
A further limiting feature lies in the relative character of the sensitivity of capacitance DLTS measurements. It is a well known fact that in such measurements: ##EQU1## in which N.sub.T is the deep level concentration
The lowest practically measureable capacitance change is about 2.times.10.sup.-5 pF (see e.g. the Hungarian Pat. No. 182.777). Taking this fact into consideration the maximum attainable sensitivity is EQU minimum of (N.sub.T /N.sub.D)=2. 10.sup.-6 ( 4)
In case of typical concentration of dopants, this sensitivity represents a detection limit level of 10.sup.10 atoms/cm.sup.3. One should, however, bear in mind that in case of higher concentration of dopants the detection limit decreases, thus in such samples the required sensitivity cannot be reached.
The above referred paper of G. Ferenczi refers also to the fact that in DLTS measurements the capture cross section can be determined by changing the width of the exciting pulses. The accuracy of such a measurement is limited by the minimum width of the exciting pulses which is about 1-2 ns and with such data higher capture cross section than .sigma.=10.sup.-15 cm.sup.2 cannot be measured, however, the largest value which should be measured is .sigma.=10.sup.-12 cm.sup.2.
This limit can be derived from the fact that in case of a capacitance measurement the sample under test should be arranged in the bridge in an isolated way i.e. unearthed. Since the measurements are carried out under varying temperatures, the practically realizable measuring arrangements are using at least 30 cm long connecting cables. With such cable lengths one cannot apply shorter exciting pulses than 1-2 ns.
Summarizing the above thoughts it can be stated that the practically attainable best parameters of capacitance DLTS measurements are as follows:
It is known in the art that very small changes of microwave absorption can be measured with a high accuracy e.g. in a microwave cavity. Such measurements are used e.g. for the examination of the paramagnetic resonance of electrons (see e.g.: G. Feher, Bell System Technical Journal 36, pp. 444-484, 1957). By means of microwave absorption measurements transient phenomena can also be detected. The measurement of lifetime of minority charge carriers by changing the microwave absorption as a function of time has been described first by Jacob et al. in the Proceedings of the IRE 48, pp. 229-233, 1960. During the lifetime measurements of minority charge carriers the change in microwave absorption due to the presence of non-equlibrum free charge carriers is detected. This method has become a widely used technique (e.g. R. I. Desi et al. Rev. Sci. Instrum. 55, pp. 1343-1347, 1984).
During lifetime measurements but also during other kinds of measurements based on microwave absorption the sample under test is not provided with contacts for electrical connections. Without electrical contacts the generated non-equilibrum carriers will move away with thermal drift velocity in the material sample and their number changes with the recombination process. The presence of free charge carriers in the sample is therefore determined by the recombination process. The above referred measurement technique, which aims at detecting the lifetime of minority carriers in the sample, is principally inappropriate for determining the time constant of thermal emission.